A Course in Local Anesthesia–Page 6-DoctorSpiller.com

local anesthesia page 1234567

Allergies to Dental Anesthetic

This is the sixth of seven pages which constitute a course in local anesthetics.  Each page stands on its own, however for a thorough understanding of dental local anesthetics the reader is advised to read the pages in order.

For patients who are allergic to local anesthetic

There are two broad classes of injectable local anesthetic.  They are the amines and the esters.  It is important to understand this when considering whether or not you are allergic to local anesthesia, since an allergy to an ester based anesthetic does not imply an allergy to the more important class of amine based anesthetics.


Esters were the first class of local anesthetic to be discovered (cocaine) and later to be synthesized (Novocaine).  They contain such drugs as Cocaine, Procaine, Tetracaine, Chloroprocaine and Benzocaine.  Today, only benzocaine is routinely used in dentistry, and its use is limited to topical application (applied with a cotton swab prior to injections or for minor procedures).  The others are used today mostly in obstetrics and for producing spinal anesthesia.

The most well known dental anesthetic in this class was Novocaine.  Novocaine. was the brand name for procaine, the very first chemically synthesized, non addictive, injectable local anesthetic ever produced.  It was invented at the end of the 19th century.  It is no longer used in dentistry partly because it has a short duration, and partly because it is highly allergenic.  High allergenicity is a trait common to all the ester based anesthetics.  In general, a patient known to be allergic to one ester anesthetic is likely to be allergic to all ester anesthetics.

The use of ester based anesthetics in the form of benzocaine topicals can be avoided entirely since 5% lidocaine gel (lidocaine is not an ester) works quite well as a topical anesthetic to numb the gums before the shot is given.


Amines were invented later.  They include Lidocaine, mepivicaine, bupivicaine, articaine, prilocaine and bupivicaine.  They all have the advantage of being non allergenic.  To my knowledge, there has never been a true, documented allergic reaction to an amine anesthetic that contained no preservatives.  Since dentists now use amine based anesthetics, and no longer use ester based anesthetics, we see almost no allergic reactions to the injectable local anesthetics.   If you have suffered allergic reactions at the dentist’s office, it is most likely that the reaction was to the topical anesthetic applied with a swab before the shot, or to the preservative used in anesthetics containing vasoconstrictors. 


Upon occasion, a patient may have a legitimate allergic reaction to an injected dental local anesthetic.  It is likely that he or she may in fact be allergic only to the bisulfite preservative used to stabilize the vasoconstrictor.  If the allergic reaction was not too serious, it may be worth trying again with either mepivicaine or prilocaine without vasoconstrictor.   Anesthetic manufactures do not use preservatives in carpules that do not also contain vasoconstrictor.

Testing for anesthetic allergy using skin tests

One of the most commonly used skin tests used by physicians to test for general allergy is called the T.R.U.E. Test®.  This is a patch test that applies 23 allergens to the skin contained in 12 polyester patches.  One of the patches contains a mixture of several anesthetics and is used to test for allergy to local anesthetics in general.  The mixture used includes two ester based anesthetics and one amine based anesthetic.  This mixture of anesthetics is called the “Caine Mix“, and most people are not aware that a positive T.R.U.E. patch test does not necessarily indicate that the patient is allergic to injectable dental anesthesia.  The patient may instead be allergic to only the ester based anesthetics (generally used only as topicals in dentistry), but not to the amines which are injected after the topical and produce the profound anesthesia necessary for dental surgeries.

What if you have had an allergic reaction to amine local anesthetics?

If you think you are allergic to dental anesthetics, the first thing you should do is to visit an allergist to see if you really are allergic to the amide based anesthetics.  You could be saving yourself a LOT of difficulties in the dental office!  (Probably, you are only allergic to the preservative used to stabilize the vasoconstrictor, or the topical that the dentist used to swab the area to be injected.  The chances are that the use of mepivicaine or prilocaine without vasoconstrictor will NOT cause an allergic reaction provided that no ester based topical is used beforehand!)

The decision to administer or receive a drug that the patient is known to be allergic to is not a trivial matter.  Even though anaphylaxis is quite rare with amine based local anesthetics, it is still possible, and both the dentist and patient must acknowledge and be prepared to deal with the consequences.  On the other hand, serious dental pain and poor dental aesthetics have real life consequences which may be just as bad for the patient as the possibility of having to deal with the effects of the allergy, no matter the consequences.

If you should decide to try dental treatment with local anesthetic, take these precautions:

The dentist should always use mepivicaine or prilocaine without vasoconstrictor.  If you have suffered allergic reactions to dental anesthesia in the past, there is a good chance that your allergic reactions were caused by the preservatives used whenever a vasoconstrictor is present.

Do not use any topical anesthetic except 5% lidocaine gel.  Benzocaine, the most popular type of topical is an ester and is likely to stimulate an allergic reaction.

Take 50 mg of Benedryl, or a similar antihistamine an hour before the dental appointment.  Benedryl (diphenhydramine)  is available at pharmacies without a prescription.

The signs of severe allergic reaction

  • SyringeVertgeneralized body rash or skin redness
  • itching, urticaria (hives)
  • broncospasm (difficulty breathing) 
  • swelling of the throat 
  • asthma
  • abdominal cramping
  • irregular heartbeat 
  • hypotension (low blood pressure) 
  • swelling of the face and lips (angioneurotic edema)

Allergic reactions can have any degree of severity ranging from minor itching to full blown anaphylaxis.  In a very serious anaphylactic reaction, the patient may experience serious difficulty breathing due to closing down of the bronchioles in the lungs or swelling in the throat area due to urticaria as well as seriously low blood pressure leading to anaphylactic shock.  This set of events, left untreated can lead to death.

Anaphylaxis is, of course the worst case scenario.  Fortunately, the majority of allergic reactions to local anesthetics are fairly mild and are easily treated with light antihistamines like diphenhydramine (Benedryl).  In a vast majority of situations, patients who have patch tested allergic to all modern local anesthetics can be safely injected for necessary dental work using an anesthetic without vasoconstrictor, provided the dentist is ready with the appropriate drugs and training necessary to combat an anaphylactic reaction in the unlikely event one should occur.

Common Non-Allergic reactions to dental anesthetics: The “Head Rush”

It is not uncommon for patients to feel nervous with a fast heart rate immediately after receiving a shot of local anesthetic.  This generally happens when an anesthetic with a vasoconstrictor is being used.  In these cases, the vasoconstrictor is entering into larger veins and going into general circulation.  The vasoconstrictor is the same as the naturally occurring “fight or flight” hormone adrenaline, and it is acting just like the release of natural adrenaline would to produce a heightened sense of awareness, nervousness and an increased heart rate.  This is a short lived reaction and is generally not dangerous.  The feeling fades after a minute or two and the patient feels much better.

Patients often mistake this “head rush” for an allergic reaction.  It is important that the patient does not mistakenly assume that they are actually allergic to dental anesthesia because the dentist has no choice but to take them at their word and the patient may find it difficult to find a dentist willing to treat them.

How to treat anaphylactic shock

The most frequent problem encountered in anaphylactic shock is swelling in the neck area which can block breathing.  This is the primary reason for death during an anaphylactic reaction!

Remember the acronym P-ABCD


Position the patient on his or her back with the feet elevated.


Maintain an airway using the chin lift-head tilt method. This is really part of first step (positioning), but in most cases, this is the only measure needed to see the patient through the emergency! If the patient is breathing on his or her own, then the next steps in the emergency protocol will be unnecessary.


If the patient is not breathing on his own, use rescue breathing like you learned in CPR class and proceed to the next steps in the emergency protocol.

Cardiovascular Assistance

Circulatory collapse in anaphylaxis is a less frequent complication, but it is absolutely essential to be prepared to counteract it.


The two drugs that dentists must have on hand to stabilize a patient in anaphylactic shock are as follows:

  • Epinephrine (adrenalin) 1:1000 subcutaneous injection.  This drug is standard in any emergency kit and counteracts all the serious effects of anaphylaxis immediately.  It opens the bronchioles allowing free breathing, increases the blood pressure counteracting shock and evens out and intensifies the heart beat.  Its effects are drastic, but short lived.  The standard dose is 1 mg given in three doses five minutes apart.
  • Benedryl (diphenhydramine) 25-50 mg injectable. This is an antihistamine and can also be taken in pill form an hour before the procedure to help prevent serious allergic reaction before it begins.  Most emergency kits contain injectable diphenhydrimine which can be administered either subcutaneously, or in the buccal fold if the dentist is more comfortable with that route.

If the patient suffers a severe allergic reaction, call 911!!  The crash team should be in your office within a short enough period to avoid major problems provided that you carry out standard emergency measures.  Note that all measures conform to the standard PABCD protocol that medical personnel learn in their basic schooling:

Check the carotid artery for heartbeat and use chest compressions if necessary. Definitive care includes includes drugs and  Defibrillation if necessary and if an AED (Automatic External Defibrillator) is available.

The following drugs are of little use to the dentist during the initial stages of the emergency since they are generally used by EMS personnel for longer term stabilization of the patient.  They are often included in standard emergency kits, but are best ignored while trying to stabilize a patient in anaphylaxis.

  • Aminophylline This drug opens blocked breathing passages.
  • Solu-cortef IV injection.  This drug is a corticosteroid and reduces the generalized allergic inflammatory reactions on a longer term basis.  It will not act rapidly enough to reverse anaphylaxis immediately, but is more of a long term remedy
  • Wyamine injection.  This drug is used to counteract hypotension (low blood pressure and shock) on a prolonged basis.

The use of Benedryl as a local anesthetic

Surprisingly, ordinary Benadryl injection (50 mg/ml=5%) can be used as a local anesthetic.  Properly diluted it can be used just like any dental local anesthetic without the danger of allergic reaction.  The major drawback is that it tends to burn on injection, and the patient should know that this is the case before getting the shot.  Most dentists have the necessary injectable in their emergency drug kits, or they can order them from the pharmacy. Start with the epinephrine (from the same concentration found in the emergency kit, (1:1000) by drawing some into a 5cc syringe and then expelling all of it. Enough remains in the needle and the syringe nozzle to bring about vasoconstriction. Next, draw 1 cc of diphenhydramine into the syringe and dilute the diphenhydramine/epinephrine mixture in the syringe to 5cc with normal saline. This produces a 1% solution of Benedryl. Dentists that place implants have saline in their surgical kits, but it too can be ordered from any pharmacy. Benedryl is not toxic even if used full strength.

Dental syringe

<==Problems arising from local anesthetics

Drug testing and local anesthetics==>

local anesthesia page 1234567